1. Field of the Invention
The invention relates to a method for controlling the beta-tin (β-Sn) crystal orientation in solder joints, suitable for substantially enhancing the reliability of microelectronic solder joints.
2. Description of Related Art
Soldering is a process in which two or more metal items (hereafter termed metallization pads) are joined together using an alloy(s) with a relatively low melting temperature (hereafter termed solder), so as to achieve an electrical/mechanical connect between two (or more) components. The conventional solder utilized in the electronic industry is a binary or a multi-component alloy, which is composed of tin (Sn) and other minor elements. This includes a tin-lead alloy, a tin-zinc alloy, a tin-bismuth alloy, a tin-indium alloy, a tin-antimony alloy, a tin-copper alloy, a tin-silver alloy, a tin-silver-copper alloy, a tin-silver-bismuth alloy and so on. A common metallization pad is primarily made of copper (Cu) or a copper alloy.
Solder joints might carry an electric current when electronic components are used in daily life. The average current density in a solder joint is inversely proportional to the square of joint diameter, and thereby high electron current stressing is usually experienced in small solder joints. The diffusing atoms/ions in the solder joints (e.g. tin or copper) upon high electron current stressing are driven to migrate along the direction of electron flow (i.e. towards the anode side of a conducting media) because of the momentum transfer between electrons and atoms resulting from an electron wind force. This current-stress-induced material transport is generally referred as electromigration. Typical electromigration-induced damages in solder joints include: (1) rapid metallization pad consumption; (2) solder deformations, e.g. voids (or valleys) and extrusions (or hillocks). Voids might be created at the cathode end of a conducting media (e.g. metallization pad or solder) whereas extrusions are commonly observed at the alternative side (i.e. the anode end) when an electron flow is imposed to a solder joint. These deformations in a solder enable to cause short circuit or disconnection of the solder joint, resulting in malfunction of an electronic device.
In order to reduce an excessive consumption in the copper metallization pads induced by electron current stressing, a nickel or nickel phosphorous (P) surface finish is usually deposited by electroplating/electroless-plating on the copper metallization pads, to prevent copper from contacting the solder directly. This is based on the fact that nickel has a much lower consumption/dissolution rate in the solder reaction system than that of copper. Additionally, nickel is capable of slowing down the growth of the fragile intermetallic compound (IMC) (e.g. copper-tin IMCs) at the solder/metallization pad interface. However, the thickness of the nickel surface finish is limited, so that the consumption of the film is commonly observed, especially for the solder joints experiencing a long-term electron current stressing treatment. When the nickel film is exhausted completely, the underneath copper is then exposed to the solder and a rapid consumption in the copper metallization pads can still occur. The exhaustion of the copper metallization pad inevitably damages the interconnections/circuits of the electronic devices and produces many fragile IMCs in the solder joints, which seriously threatens the electrical/mechanical reliability of electronic devices.
Electromigration in solder joints can also be alleviated by adding silver into the solder. Nonetheless, the silver is realized to be a noble metal. When the solder is doped with a large amount of silver, the price of such a solder material is therefore increased. Additionally, it is stated in “Journal of Alloys and Compounds, 352, p. 226, 2003” that when the content of silver in an solder alloy is more than 3.2 wt. %, many Ag3Sn plates are easily created in the solder matrix, and deteriorate the mechanical properties of the solder.
Therefore, in order to (1) prevent metallization pads from being greatly consumed; (2) prevent the solder deformations; (3) prevent the generation of Ag3Sn plates; and (4) reduce the material/manufacturing cost, to re-assess the composition of the Ag-containing solder has become essential in the manufacture of microelectronics.